The use of semi-permeable membranes to separate gas mixtures has become a well known technique in the production of industrial gases. Known plants for the separation of gas mixture by such membranes are constructed so as to present a large surface area of membrane to the gas mixture to be separated. For example, such plants may employ a multitude of identical, elongate, hollow fibres which are formed of a suitable semi-permeable membrane and which extend in parallel to one another. The fibres are appropriately mounted in a pressure vessel. The gas mixture to be separated is fed into the pressure vessel at or near one end outside the fibres. It flows longitudinally of the fibres. The insides of the fibres are maintained at a pressure lower than that which obtains on the outside of them. The components of the gas mixture diffuse through the membranes at different speeds. A fast permeating component passes more and more to the lower pressure side. Accordingly, the gas on the outside of the fibres (high pressure side) becomes richer in the slower permeating component as it flows along the outside of the fibres, and a product gas, enriched in the more slowly diffusing component, may be withdrawn at pressure from the end of the pressure vessel opposite that at which the feed gas is introduced. The permeate gas is enriched in the faster diffusing component. The permeate gas is withdrawn from the inside of the fibres at the same end as that at which the feed gas is introduced.
The performance of the membrane material may be described by two properties, namely its permeability (or flux) and its selectivity. The flux or permeability is basically the rate at which a permeable component of the mixture diffuses through the membrane. Its absolute value depends upon the thickness and surface area of the membrane, the pressure difference across the membrane and the ambient temperature, among other factors. The selectivity of the membrane determines the ratio of the permeabilities of the two components of the gas mixture to be separated. It is therefore desirable that in any separation the membrane has both a high permeability and a large selectivity.
The requirements of an industrial process for a particular gas are often stated in terms of the purity of the gas and its flow rate. It is thus desirable for any commercial apparatus for supplying the gas to be capable of producing the product at a predetermined purity (or maximum tolerable purity) and a predetermined flow rate. Conventionally, when an apparatus using semi-permeable membranes is used to supply nitrogen by separating it from air, the incoming air, after purification, is supplied at a constant superatmospheric pressure, while the permeate gas typically flows out of the vessel at approximately atmospheric pressure. The product nitrogen is withdrawn through a flow control valve whose setting determines the purity of the product. The effect of reducing the size of the passage through the valve is to reduce the flow rate of the gas over the membranes and hence increase the average residence time of each gas molecule within the separation vessel. Accordingly, the oxygen molecules are given more opportunity to diffuse through the membrane, and a purer product is given. By the same token, increasing the size of the passage through the valve will increase the flow rate of product but also increase its impurity level as oxygen molecules are given less average time to diffuse through the membranes. Thus, in commercial practice, the flow control valve can be given a particular setting to give product gas at a given purity and given flow rate.
In practice, however, the apparatus will not produce product gas of the same purity day in and day out during operation over a prolonged period of time. There are four factors which tend to cause the purity of the product gas to vary. The first of such factors is the temperature to which the membrane is subjected. The higher the temperature the greater the rate of permeation of the components through the membrane. If the membrane vessel is supplied with an air feed at a constant flow rate, for example, from a dedicated compressor, then an increased permeability will increase the purity of the product but reduce its yield. Further, if a temperature variation is the result of changing ambient conditions, and the air (in the case of air separation) is supplied by a compressor, increasing temperature will decrease the mass flow rate of air delivered by the compressor, so the tendency of increasing temperature to give product nitrogen at a lower flow rate will be amplified.
A second factor affecting the performance of semi-permeable membranes is the effect of contaminants in the gas mixture. Although, in commercial practice, care is taken to ensure a supply of clean gas to the high pressure side of the membranes, even the cleanest stream tends to contain some contaminant vapours or even tiny particles which over a period of time may lodge on the membrane material and cause its permeability to decrease. Such contamination will have the effect of tending to increase the impurity level in the product gas. A third factor is varying barometric pressure. This factor can be particularly important if the membrane vessel is so operated that the waste gases are withdrawn at atmospheric pressure. In practice, atmospheric pressure can vary up to 5% either side of the mean of 760 mm of mercury. Although the net effect of such variations is reduced by using a relatively high air supply pressure, their effect cannot be entirely eliminated. The fourth factor is the tendency for the membrane materials themselves, typically being organic polymers, to undergo an ageing effect over their life time in the membrane vessel. Ageing is not necessarily a relatively slow phenomenon which manifests itself only after a period of years. The ageing effect can be exponential in character with the major change occurring in the early part of the membrane's operational life.
Although the above factors may be mitigated by appropriate adjustment of the flow control valve by the operator, most membrane gas separation plants are designed for unattended operation. Moreover, effects such as ambient temperature changes can be fairly rapid and occur over a few hours which would make necessary frequent operator attention to the valve setting. Various methods have been suggested to compensate for these factors, such as using a temperature control device on the feed air to the membrane vessel, either to refrigerate it or heat it, but in any event keeping it at a stable temperature. However, the operation of these devices is complicated and they are still inadequate to give totally stable operating conditions. Nor do they compensate for longer term loss of membrane performance as a result of ageing or contamination: nor do they compensate for a varying barometric pressure.
It is the aim of the present invention to provide a gas separation apparatus which makes use of semi-permeable membranes and which has control means able to be operated automatically to mitigate fluctuations in product purity that would otherwise be caused by the factors discussed above.